The present invention relates to an excitation circuit having a piezo-electric sound generator, power supply terminals therefor, an inductance associated with the piezo-electric sound generator, and a switching unit for interrupting power feed to the piezo-electric generator.
It is known from FIG. 5 of German Pat. No. 28 23 155, FIG. 1 of British Pat. No. 2,104,257A, and FIG. 1 of British Pat. 2,104,273, all incorporated herein by reference, to employ an excitation circuit as shown in FIG. 1 herein for exciting oscillations of a piezo-electric sound generator. The piezo-electric sound generator 1 and a switch 5 lie in series with the terminals 2 and 3 for the power supply. The inductance 14 is charged with electrical energy as soon as an electrical voltage is present between the terminals 2 and 3 and the switch is closed. When switch 5 is opened, through actuation by a control means 6, the electric current flowing through the inductance 14 suddenly collapses. This produces an electrical impulse collision excitation for the sound generator 1 which is thereby induced to oscillate. It is not only the oscillation having the fundamental wave resonance of the sound generator 1 which is excited. Also other oscillation modes are excited. This is a disadvantage of this known circuit. A further disadvantage is that relatively high electrical voltage must be applied between the terminals 2 and 3 in order to generate significant acoustic power.
The prior art discussed above with reference to the publications and also from German Pat. No. 27 07 524, incorporated herein by reference, has been improved by providing a rectification of the voltage generated with the assistance of the inductance and to feed the transducer with this d.c. voltage. In order to achieve electro-acoustical oscillations of the dc-fed transducer, however, the known switch is driven with the prescribed oscillation frequency of the transducer, i.e. induced oscillations of the transducer are generated. But the problem thus arises of being able to govern the mutual tuning of this control frequency and the natural frequency of such a transducer, the latter varying as a function of the respective operating conditions. In order to resolve this problem, German Pat. No. 2,219,761, incorporated herein by reference, provides for the additional employment of a feedback, this involving considerable added expense.